Offshore exploration and production of hydrocarbon resources has for many years been performed from floating marine vessels, such as so-called SPARs, semi-submersible platforms, and purpose-built ships, in general classified as FPS or FPSOs for permanent installations. An important aspect in offshore drilling operations is the use of mobile drilling units, typically referred to as MODUs. Traditionally, all such vessels have been moored by steel chains and/or steel ropes, extending between the vessel and seabed anchors for safe station-keeping. As operations over the years have been shifted into deeper waters and thus requiring longer mooring chains, the weight of these mooring chains has become a major design parameter that the operators seek to reduce. The use of synthetic fibre ropes has therefore become more prevalent, in order to reduce overall weight, reduce fatigue characteristics and eliminate corrosion typically associated with metal-based mooring lines.
Synthetic fibre ropes for offshore mooring purposes normally comprise a core (which typically is made up of a number of polyester strands and bundles) enclosed by a braided jacket. Materials commonly used in making synthetic fibre ropes are Polyester, Aramid, and ultra high-molecular weight polyethylene (UHMWPE), and in limited cases nylon. Most synthetic fibre ropes used as mooring lines range from slightly positive to slightly negative buoyancy.
Although a synthetic fibre rope is an attractive alternative to a conventional steel mooring chain due to its low weight, it also has certain disadvantages, for example susceptibility to mechanical damage induced by handling, sharp objects and abrasion.
During installation (i.e. connection between seabed anchor and floating vessel), portions of the synthetic fibre rope are in direct contact with anchor handling vessel (AHV) decks, and often dropped onto, and/or dragged along, the seabed, whereby it might be damaged by gravel, rocks, or other sharp objects (decks, sharp steel edges, etc.).
During operation (i.e. when the fibre ropes have been installed), the lower portion of the mooring line is touching and rubbing against the seabed. Therefore, a traditional fibre rope may have limited use in this lower portion of the mooring line, and a conventional mooring chain is used instead—connecting to the fibre rope at a safe distance above the seabed.
The installed fibre rope is also exposed to objects that are dropped into or dragged through the water, which may easily cut into and damage the rope. A major threat to fibre ropes is trawl wires from either pelagic trawling or on-bottom trawling. Trawlers may regularly fish in the vicinity of either permanent or MODU installations, an activity that substantially increases the risk of damage to installed mooring lines. If a steel wire dragged across a fibre rope, the forces may be of such magnitude that the wire virtually saws through, and cuts, the fibre rope.
The prior art includes WO 2011/102730 A1, which describes an anchor spread for mooring a marine vessel, having a seabed anchor chain connected to a fibre rope. The fibre rope is prepared by being packed to a bundle or coil arranged in a protective container on the seabed. The fibre rope is pulled out of the container at a predetermined pulling force, and picked up and connected to the anchor winch chain.
The prior art also includes WO 98/50621 A1, which describes a synthetic cable used for the anchoring of floating platforms in offshore oil production. A layer to protect the cable core against ingress of particles comprises a strip of polymer material placed in helical fashion between the core and the cable's outer braided protective layer. The helical layer permits passage of water but prevents the passage of particles towards the core.
The prior art also includes WO 2013/148711 A1 (also published as US 2013/0247534 A1), which describes a rope having a cut-resistant jacket which includes a core comprised of a plurality of sub-ropes. The sub-ropes are made of fibres of a synthetic material, such as polyester, nylon, polypropylene, polyethylene, aramids, or acrylics. A cut-resistant jacket surrounds the core and is made from a material that has increased strength and/or abrasion resistance over the material of the core. The cut-resistant jacket may comprise steel wires and may further comprise braided steel wires or rope. The braided steel wires or rope may be covered with a plastic material for increased corrosion resistance. A filter layer, for preventing particles larger than a certain size from entering the core, may be disposed between the core and the cut-resistant jacket and may be wrapped around an outer surface of the core prior to the cut-resistant jacket being formed.
The prior art also includes WO 97/09481, which describes a buoyant rope assembly having a central rope, e.g. of nylon, a plurality of flotation elements of closed cell foam, e.g. of polyethylene, buffer elements of open cell foam, e.g. of polyurethane, arranged between and flush with the flotation elements, and a protective outer layer, e.g. of polyurethane.
It is therefore a need to improve the durability and reduce the vulnerability of fibre ropes used as mooring lines.